Lists of objects

Storing objects in a list

The ease of use of objects and the utility of lists mean that we'll often combine the two.

const books = [
  {
    title: "The Handmaid's Tale",
    author: 'Margaret Atwood',
    year: 1985,
    isbn: '9780099740919',
    isAvailable: false
  },
  {
    title: 'Oryx and Crake',
    author: 'Margaret Atwood',
    year: 2003,
    isbn: '9780385721677',
    isAvailable: true
  },
  {
    title: 'Brave New World',
    author: 'Aldous Huxley',
    year: 1932,
    isbn: '9780060929879',
    isAvailable: false
  },
  {
    title: 'The Geometry of Type',
    author: 'Stephen Coles',
    year: 2013,
    isbn: '9780500241424',
    isAvailable: true
  },
  {
    title: 'The Design of Everyday Things',
    author: 'Don Norman',
    year: 1988,
    isbn: '9780465050659',
    isAvailable: true
  }
]

We can access the properties of each book in the same way as we would for a single object.

js

console.log(books[0].title)
console.log(books[2].author)
console.log(books[books.length - 1].year)

output

The Handmaid's Tale
Aldous Huxley
1988

Using loops

We can use our knowledge of loops to solve problems on arrays of objects. For example, suppose we need to count how many books were published in the 20th century.

js

let count = 0

for (let book of books) {
  if (book.year < 2000) {
    count++
  }
}

console.log(count)

output

3

Transforming with map

Oftentimes, we'll want to transform the data in some way. A common thing to do is to select particular properties from each object.

js

const bookTitles = books.map(book => book.title)
console.log(bookTitles)

output

[
  "The Handmaid's Tale",
  'Oryx and Crake',
  'Brave New World',
  'The Geometry of Type',
  'The Design of Everyday Things'
]

Filtering data

Let's say we want to find all the books that are available.

js

const availableBooks = books.filter(book => book.isAvailable)
console.log(availableBooks)

output

[
  {
    title: 'Oryx and Crake',
    author: 'Margaret Atwood',
    year: 2003,
    isbn: '9780385721677',
    isAvailable: true
  },
  {
    title: 'The Geometry of Type',
    author: 'Stephen Coles',
    year: 2013,
    isbn: '9780500241424',
    isAvailable: true
  },
  {
    title: 'The Design of Everyday Things',
    author: 'Don Norman',
    year: 1988,
    isbn: '9780465050659',
    isAvailable: true
  }
]